Catalytic reforming of naphtha involves a number of competing processes or reaction sequences. These include dehydrogenation of cyclohexanes to aromatics (benzene), dehydroisomerization of alkylcyclopentanes to alkylaromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, hydrocracking of paraffins to light products boiling outside the gasoline range, dealkylation of alkylbenzenes and isomerization of paraffins. Some of the reactions occurring during reforming, such as hydrocracking which produces light paraffin gases, have a deleterious effect on the yield of products boiling in the gasoline range. Process improvements in catalytic reforming thus are targeted toward enhancing those reactions effecting a higher yield of the gasoline fraction at a given octane number.
It is important that a catalyst exhibits the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended function in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these three parameters are defined as follows: (1) Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level, with severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity typically is characterized as the octane number of the pentanes and heavier (“C5+”) product stream from a given feedstock at a given severity level or conversely as the temperature required to achieve a given octane number. (2) Selectivity refers to the percentage yield of petrochemical aromatics or C5+ gasoline product from a given feedstock at a particular activity level. (3) Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time or of feedstock to achieve a given C5+ product octane, with a lower rate of temperature change corresponding to better activity stability, since catalytic reforming units typically operate at relatively constant product octane. Selectivity stability is measured as the rate of decrease of C5+ product or aromatics yield per unit of time or of feedstock. Hydrothermal stability refers to the ability of a catalyst to withstand extended conditions associated with commercial operation and periodic regeneration to remove accumulated coke deposits. Coke deposits are a well-known cause of catalyst deactivation and are typically removed through exothermic combustion. Such periodic regeneration most frequently results in surface area decline and reduced support capacity to hold anions such as chloride. Thus, a steam treatment test to study surface area decline can be useful in simulating long-term hydrothermal stability over prolonged periods of time.
Programs to improve performance of reforming catalysts are being stimulated by the reformulation of gasoline and related refinery demands for constant hydrogen supply. Gasoline-upgrading processes such as catalytic reforming must operate at higher efficiency with greater flexibility in order to meet these changing requirements. The major problem facing workers in this area of the art, therefore, is to develop catalysts with more stability, activity, and selectivity.
U.S. Pat. No. 2,890,167 to Haensel broadly discloses a gasoline reforming process in the presence of a catalyst containing a platinum group metal and a phosphorus component.
U.S. Pat. No. 4,306,963 to Johnson discloses a thermally stabilized halide-promoted, supported noble metal reforming catalyst that uses a minor amount of silica with a mull of alumina prior to extrusion and calcination.
U.S. Pat. No. 4,483,767 to Antos et al. discloses reforming over a catalyst having a platinum group composition and also containing phosphorus. Such a catalyst shows best results with about 0.5 wt-% phosphorus and about 1.0 wt-% chloride on gamma-alumina.
U.S. Pat. No. 5,972,820 to Kharas et al. discloses methods of stabilizing crystalline delta phase alumina compositions, including specific compositions with an effective lower limit of 1.0 wt-% phosphorus, silicon, germanium, or arsenic oxides.
Moreover, a catalytic reforming process with a chlorided catalyst that can effectively retain sufficient chloride has longer over-all life with reduced chloride consumption (and consequent corrosion resistance) and improved economics than a chlorided catalyst with excess stabilizing component that cannot retain sufficient chloride. Over-all life of a reforming catalyst is typically considered to be near the end of its useful life once the surface area has declined below 150 m2/g. Excessive chloride consumption occurs with lower levels of surface area where catalysts lose part of their ability to retain chloride species, and thus a minimum useful chloride retention level for a reforming catalyst is typically considered to be about 0.8 wt-%. Such chloride provides acidity function to the catalyst which facilitates isomerization and cracking reactions which participate in allowing the catalyst to transform a low octane feed into a high octane product.
As heretofore mentioned, processes of catalytic reforming continue to be developed that utilize improved catalysts for the increased economical production of gasoline and hydrogen. Control of catalyst acidity is an important part of such processes.
U.S. Pat. No. 3,287,253 to McHenry, Jr. et al. discloses a selection and use of three specific catalysts in three zones to provide an improved naphtha reforming process. The first catalyst has low halogen retention in order to avoid formation of cracking sites by using a non-acidic support, while the second catalyst has an acidic support such as silica-alumina, and the third and final catalyst has a platinum-alumina-chloride complex. Chloride injection and removal is disclosed around the third stage catalyst.
U.S. Pat. No. 3,846,283 to Rausch discloses a catalytic reforming process for a gasoline fraction with a bimetallic catalyst having a platinum group component, a tin component, and a halogen component that uses a halogen additive in an amount of about 0.1 to 100 wt-ppm of the gasoline fraction.
U.S. Pat. No. 3,864,240 to Stone discloses an integrated system with a fixed-bed catalyst system and a movable, gravity-flowing catalyst system. The movable catalyst is subjected to a chlorination zone when it moves into a regeneration section. This system is similar to the integrated systems disclosed in AM-96-50, “IFP Solutions for Revamping Catalytic Reforming Units” by Gendler et al. This system is also similar to the integrated system disclosed in AM-03-93, “PLATFORMING Technology Advances: CYCLEX System for Increased Hydrogen Production from a Fixed-Bed Reforming Unit” by Peters.
U.S. Pat. No. 4,832,821 to Swan discloses a catalytic reforming process where the level of halide in a multiple reactor system is maintained in each reactor by injecting into each reactor a mixture of water and halide at a ratio of 20:1 to 60:1.
U.S. Pat. No. 5,837,636 to Sechrist discloses a method of reducing chloride emissions from a catalyst regeneration process used with catalytic reforming by sorbing a portion of the chlorides from an effluent stream onto catalyst particles. The method captures and returns to the process the chlorides that would be lost to the process and that would need to, be replaced by the injection of make-up chlorides.
U.S. Pat. No. 6,558,532 to Lin et al. discloses periodically contacting a reforming catalyst with an organic chloride in an effective amount to restore at least a portion of catalyst activity.